「植物看得見你」公開課筆記／2.7 Molecular Biology of Plant Vision

Several decades ago, it was discovered that one of the first things that happens when a plant is illuminated with blue light, is a change in a specific protein. One protein in the membrane of the plant cell becomes phosphorylated. What that means is that phosphate is added to this protein.

Scientists add radioactive phosphate to the plant, isolates the proteins, and then see which proteins have become radioactive. What we can see here in this picture, this is actually showing a specific protein that when it's in the dark, it has a certain level of phosphorylation and we see that by the width, or the size of this band in the gel. And here are the same protein after it's illuminated with blue light, it gets much stronger signal. That means there's a lot more phosphate on this protein. And this is induced only by the blue light, okay? So here we have a protein that becomes phosphorylated in response to blue light. Now in all of biology phosphorylation is one of the ways that proteins signal to each other. And this is true for all of Animal biology, bacterial biology, plant biology.

The mutants that were blind to blue light, it actually turns out that there was more than one mutant that were blind. These mutants were called NPH at the time for non phototropic. There's actually four mutants isolated. Here we could see the normal, the wild type, which is bending to the light, which is coming from the side here. And mutations in four different genes and nph-1, nph-2, nph-3, and nph-4, are all blind to the light. These are mutants in different genes but they all look the same, they're color blind. In other words, if I give the blue light Is the same protein still phosphorylated.

In this experiment, we can see that the results are different depending on the mutant. For example, in nph-1, we can clearly see that there is no change in phosphorylation in response to the light. In the wild type, we see that when it's illuminated with a blue light, there's a huge change in the amount of this band, a lot more phosphorylation. In nph-1, a mutant that's blind to blue light. There is no phosphorylation, but on the other hand, in the other mutants, in nph-2 or nph-3 or in nph-4, this protein is still phosphorylated.

We have two classes of proteins, two classes of mutants. One mutant, nph-1, affects the original phosphorylation. Whereas mutants in the other sub-units, the phosphorylation still occurs, but the plant is still blind to the light.

10 or 20 years, well we've now cloned the genes, my colleagues have cloned the genes. We know what they're doing, here's what's happening. So if we now take all the data that's accumulated for these phototropic mutants, here's what we now know. This is our paradigm right now, a blue light signaling for phototropism. We now know that NPH1 encodes a protein which we now call phot 1, or phototropin 1. This is the blue light receptor necessary for phototropism.

When phot 1 absorbs blue light, it becomes phosphorolated. A phosphate group is attached to it. Once volt one is phosphorolated, it can then interact with NPH3, physically touch each other within the cell. And then NPH3, somehow takes the information from phot 1, and transduces this signal further into the plant.

The signal from NPH3 then effects a plant hormone that's called auxin. We're gonna learn more about auxin later in this class. But auxin is one of the main hormones that is important for phototropism.

The auxin signal is then modulated by NPH4, which itself is a transcription factor. What that means, this is a protein which directly effects which genes are turned on and which genes are turned off, finally leads to phototropism.

So we could see that starting from a simple genetic screen which isolated various mutants that are all blind to blue light. We isolated a photoreceptor.

If this photoreceptor doesn't act or doesn't exist, the plant is blind to blue light. We isolated one of the proteins involved in the signal transduction, in the transfer of the signal after light perception. If this signalling component is missing, then you don't get phototropism. And we also isolated a protein directly involved in the regulation of which genes are turned on and turned off which will be very far downstream in the pathway. If this doesn't exist, the plant will be blind to blue light.

So we can affect light signaling at different levels, at the level of perception, at the level of transduction or at the level of the response. Just like in humans we can be blind because of a problem in the photoreceptor, a problem in the transduction in the nerves, or a problem of the response in the brain. All three defects lead to blindness, but they're caused by different problems.

Blue light effects many process in the plant, but it also affects when plants open the small holes on the leaves which are called stomata, it affects how chloroplasts, the part of the plant that does photosynthesis, actually move within the cell, how leaves follow the sun during the day. Blue light responses maximize the ability of a plant to carry out its photosynthesis.

This thing about solar tracking is very important because the first class I ever learned in signaling was taught by Professor Dove Caldwell in Hebrew University, who specialized in understanding how plants change the position of their leaves in response to the direction of the sun. Recently a book was published called the restless plant is written by him. Dove Koller is the father of Professor Dofnick Koller, who was one of the founders of Corcyra.